1. Field of the Invention
This invention relates to medical devices and more particularly relates to a process for modifying the chemistry of a particular region of the surface of the device.
2. Background
Blood samples are routinely taken in evacuated tubes, such as glass VACUTAINER.TM. tubes (Becton Dickinson and Company). One end of a double-ended needle is inserted into a patient's vein. The other end of the needle then punctures a septum covering the open end of the VACUTAINER.TM. tube so that the vacuum in the tube draws the blood sample through the needle into the tube. Using this technique a plurality of samples can be taken using a single needle puncture of the skin.
Plastic tubes have also been proposed for blood collection. Plastic offers a number of advantages such as lower breakage than glass tubes, less weight in shipment, and easier disposal by incineration.
During sample collection, the blood comes into contact with the interior surface of the collection tube. The nature of the blood surface interaction is complex. For example, the blood clotting cascade is activated by contact of the blood with a wettable glass surface and the formed clot does not adhere to the glass, making it easy to separate clot from blood plasma by centrifugation. However, because the clot is non adherent, it is subject to resuspension in part or whole into the plasma layer during handling or transportation. Thus, in glass tubes, the same surface properties that lead to easy separation of clot from the serum also allow the clot to move freely in the tube.
Plastic tubes generally have a nonwettable surface and do not activate the clotting process efficiently. Clots formed in these tubes may be quite gelatinous relative to those in glass. These gelatinous clots adhere tenaciously to plastic surfaces and do not allow for clean separation of serum from clot in conventional hematological centrifuges. However, the adherent clot is not easily disrupted by ordinary handling procedures. Thus, although plastic surfaces do not cleanly separate serum from clot, the adhesion of the clot to the plastic surface and its mechanical stability is a positive feature not available in glass tubes.
Thus, the ideal blood collection tube for serum separation would exhibit both glass-like and plastic like surface activity. In the present disclosure, the term glass-like is used to describe a surface which is substantially wettable by aqueous liquids, which initiates clot formation and which is non-adherent to the formed clot. The term plastic-like is used to describe a surface which is substantially nonwettable by aqueous liquids, which does not initiate clot formation to any significant degree, but which is highly adherent to clot material.
Various methods have been proposed for modification of glass and plastic surfaces. In one method, plastic surfaces are coated with a detergent material to render them more glass-like. This approach has the disadvantage of adding a soluble foreign material which contaminates the serum and may interfere with subsequent blood analysis. Another approach has been to treat surfaces with a gas plasma to modify either the hydrophilic or hydrophobic character of the surfaces. In these approaches, the entire interior surface of the tube is substantially modified to interconvert glass-like and plastic-like surfaces.
Formation of a hydrophobicity gradient on a polydimethylsiloxane film is taught by Golander et al. in Biomaterials 11,32 (1990). The film surface is exposed to an oxygen plasma generated in a planar diode system for various lengths of time as regulated by a cover movably positioned between the plasma and the polymeric surface.
Wettability gradients are formed on silicon plates by Elwing et al. in Advances in Colloid and Interface Science, 32,317 (1990). The gradients are formed by diffusion-induced silylation using dichlorodimethyl silane.
There is need in the art of blood collection for a tube which promotes clot formation, permits clean separation of clot from serum and enhances strong adherence of the formed clot to minimize mechanical remixing of clot material with serum.